In-situ high-pressure and high-temperature spectroscopic studies of phengite in ultrahigh-pressure eclogite: implications for water transport during ultra-deep continental subduction

Pressure and temperature responses of natural phengite [K 0.93 Na 0.03 (Al 1.46 Mg 0.45 Fe 0.09 )(Si 3.59 Al 0.39 Ti 0.02 )O 10 (OH 1.94 F 0.06 )] in ultrahigh-pressure eclogite from the main hole of the China Continental Scientific Drilling Project (CCSD), the Dabie-Sulu orogenic belt have been studied using in-situ high-pressure mid-infrared and high-temperature Raman spectroscopic measurements up to ~ 20 GPa and 800 °C, respectively. Linear positive pressure dependences were observed for the infrared absorption bands associated with the aluminosilicate vibrations up to ~ 19 GPa, indicating the steady compression of the structure framework. The frequencies of the O–H stretching doublet bands, initially at 3601 and 3626 cm −1 , displayed linear downshifts up to 16.6 GPa at − 2.02 and − 2.72 cm −1 /GPa, respectively, implying high stability of the hydroxyl groups under compression. In the high-temperature Raman spectra, the bands initially centered at 265, 420, 703 cm −1 , and the O–H stretching modes at 3620 cm −1 exhibited modest linear negative shifts with increasing temperature up to 800 °C. Comparisons between experimental results of the present study and those of the previous studies make it plausible that phengite with a higher Si content, i.e., a higher tetrahedral Si/Al ratio, would have higher stabilities under both high pressure and high temperature, and is likely to transport water to greater depths during subduction processes.